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R. Berzin* and the URSEIS Working Group; *SPETSGEOFIZIKA, Moscow, 107140, Russia
Results of the URSEIS '95 integrated seismic experiment provide fundamental new data revealing the lithospheric architecture of an intact Paleozoic collisional orogen. Hybrid-source seismic reflection and refraction data image a crustal-scale collisional seismic fabric and pronounced crustal root preserved since Paleozoic time, and a 2-D picture of upper mantle structure, perhaps down to the base of the lithosphere. The lack of evidence for syn- or late-orogenic extensional collapse, and the preservation of the orogenic structural fabric as well as the crustal root, suggest that the Urals do not conform to existing models of post-orogenic evolution. This history may be a consequence of an incomplete or łarrested˛ collisional process, and has led to preservation of the largest continental landmass on earth. Current models for orogenic evolution of the continental crust typically invoke syn- or post-orogenic extensional collapse, driven perhaps by gravitational instability of the crustal column and/or delamination of the lower crust/mantle lithosphere. The literature now contains numerous examples (Himalayas, Alps, Cordillera, Variscides, Caledonides, Appalachians) of compressional belts in which large-scale extension has played a significant role in the later stages of orogenic development. Such concepts are a modern elaboration of Wilson's (1966) articulation of the recurrent opening, closing, and re-opening of ocean basins in what has become widely known as the Wilson Cycle of plate tectonics. Orogenic belts that formed through closure of an ocean basin and subsequent continental collision may later serve as the focus for continental rifting and formation of a new ocean basin. An expected consequence of such wholesale re-equilibration of the lithosphere is a return to pre-orogenic or even thinned continental crust (30-40 km). Such "normal" or "thinned" crustal thicknesses now characterize many older orogens, including the Paleozoic suite of the Variscides, Caledonides, and Appalachians. The URSEIS Ś95 seismic survey consists of three main components: (1) a 465-km long near-vertical incidence vibroseis-source reflection survey, (2) a coincident near-vertical incidence explosive-source survey, and (3) a 340-km explosive-source wide-angle reflection/refraction survey including a cross line and two off-line fan-recording shots. The transect extends from the East European platform in the Uralian foreland (Sterlitamak) to the West Siberian basin (Nikolaevka), crossing the Bashkirian foreland fold and thrust belt, the Kraka ophiolite, the Main Uralian fault (suture zone), a collage of oceanic and microcontinental terranes in the hinterland, and the main axis of orogenic magmatism. Acquisition of these data was accomplished during the summer and fall of 1995 (June-Nov). Integration of data from these experiments provides one of the most complete seismic images ever obtained for the continental lithosphere. Participants in this project include Spetsgeofizika (Moscow), Bashneftegeofizika (Ufa), and the Bazhenov Geophysical Expedition (Scheelite) from Russia (with funding from ROSCOMNEDRA) and from the West the German DEKORP group (GFZ-Potsdam), Cornell University (USA), and ICTJA (CSIC-Barcelona, Spain), and represents an international collaboration of unprecedented scale in deep seismic profiling. The planning, funding, acquisition, and analysis for this experiment have all been carried out jointly, and the success of this collaboration may serve as a useful model for future international projects.
A LITHOSPHERE-SCALE SEISMIC IMAGE OF THE SOUTHERN URALS FROM URSEIS '95 EXPLOSION-SOURCE REFLECTION PROFILING
J.H. Knapp* and the URSEIS Working Group; *INSTOC, Cornell University, Ithaca, NY 14853, USA
New explosive-source deep seismic reflection data from the southern Ural Mountains of central Russia, collected as part of the international URSEIS Ś95 Project, provide a lithosphere-scale image of the central Eurasian plate, and reveal the deepest reflections (35-45 s; ~130-170 km) yet imaged with the controlled-source seismic reflection method. The CMP reflection data display laterally variable reflectivity at the base of the crust (Moho) that deepens beneath the central part of the profile, documenting a crustal thickness of ~60 km beneath the axis of the orogen. Sub-crustal reflections are imaged at several depths beneath the Uralian hinterland, demonstrating a pronounced mantle reflection fabric that has either been preserved since Late Paleozoic collision, or represents younger features of the mantle lithosphere developed beneath a crust which has been undisturbed structurally since Paleozoic time. Mantle reflection fabrics, imaged primarily beneath the Uralian hinterland, can be categorized into three principal features, including: (1) gently W-dipping reflections at 35-45 s TWT (~130-170 km) (Nikolaevka Reflection Sequence; NRS), (2) a sub-horizontal band of reflections from 22 to 24 s TWT (~85 km) (Alexandrovka Reflection Sequence; ARS), and (3) a diffuse E-dipping fabric which characterizes much of the upper mantle in the east. In the eastern part of the profile, the Nikolaevka Reflection Sequence (NRS) consists of a series of reflections between 35-45 s TWT that can be traced for more than 75 km. These reflections include four west-dipping to subhorizontal reflection packages of ambiguous polarity and a signal-to-noise ratio of 2:1. Additionally, discontinuous, subhorizontal events consisting of two bands of reflectivity occur at 55 s (~200 km) in the western part of the profile. There remain several possible interpretations for the NRS, including the base of the thermal lithosphere, a low velocity (shear?) zone, fine-scale compositional variation, a relict W-dipping subduction zone, or fluids (magma?) trapped within the mantle. Low velocity zones have been identified at similar depths within central Eurasia on Peaceful Nuclear Explosion data and the NRS may correlate with these, implying a regional (>2000 km ) extent for this mantle signature. Since the polarity of the NRS reflections has not yet been determined with our existing P-wave data, it is not possible to verify whether these reflections represent low or high velocities with respect to the surrounding mantle. However, the depth of the NRS is consistent with estimates for lithospheric thickness throughout Europe and may therefore represent the base of the lithosphere in the Southern Urals. The ARS consists of a thin (<0.25 s), complex band of sub-horizontal reflections at ~24 s TWT, continuous for at least 50 km across strike. While the traveltime of the ARS matches that expected for a shear wave reflected from the Moho (SmS), the reflection appears to have a velocity more appropriate to a P wave (PxP). Additionally, comparison with the overlying west-dipping Moho suggests this event is not a simple P-wave multiple. The ARS may represent a mantle shear zone (subduction-related?) developed during assemblage of the oceanic and micro-continental terranes in the Uralian hinterland. Diffuse reflection fabrics are observed in the upper mantle on both the eastern and western parts of the profile, and upon migration remain at upper mantle depths. Dipping reflections in the mantle from 20 to 40 s may be related to remnant fabrics of Paleozoic subduction.
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